The present invention relates to an optical transmission system composed of a number of switched optical network nodes having a number of unidirectional terminating units, in which each unidirectional terminating unit of a network node is connected to a unidirectional terminating unit of a further network node via a unidirectional connection path section. In addition, the present invention relates to a method for setting up at least one bidirectional connection path from a first optical network node to an N-th optical network node via at least one connection path section in such an optical transmission system.
In optical transmission systems, particularly wavelength division multiplexing (WDM) transmission systems, optical data signals are transmitted at different data rates. In order to implement high transmission capacities, a number of optical signals having different wavelengths, such as WDM signals, are transmitted as a single optical transmission signal or data signal. For this purpose, optical transmission systems have optical cross connectors, or optical network nodes, which are, for example, connected in series and which are connected to one another via, for example, point-to-point connections using optical connecting fibers. In such a case, optical connections are set up, maintained and released via a number of optical cross connectors or optical network nodes.
However, the operators of optical transmission networks and systems wish to increase the flexibility of the adaptation of such optical transmission systems to dynamically changing traffic volumes or traffic requirements. For this purpose, switching matrices are provided in the optical network nodes which permit flexible switching over of the optical data streams or optical data signals on the basis of individual wavelengths. This is referred to as dynamic “wavelength routing”.
By automating this “optical channel layer,” i.e., by providing an automatically switched optical transmission system (automatically switched optical network) (ASON), the restoration times and the connection setup times in the case of a fault are considerably reduced.
In particular, the setting up of bidirectional connections in an automatically switched optical transmission system (ASON) is of interest for the operators of such optical transmission systems. The physical layer which is provided for this is implemented in such an optical automatically switched transmission system (ASON) via unidirectional optical connection paths. To set up a bidirectional optical connection path, two unidirectional optical connection paths are therefore necessary; i.e., two separate optical connection paths are provided for the forward and backward transmission directions. For this purpose, a first unidirectional connection path is first set up in the forward direction and then a second unidirectional connection path is set up in the backward direction. As a result, a considerable degree of signaling complexity is required to set up bidirectional connection paths in optical transmission systems (ASON).
In the article “Control of Lightpaths in an Optical Network”, Sid Chaudhuri, et al., Optical Internetworking Forum, a method is described, for example, in which a bidirectional optical connection path is set up in two separate stages. Here, a unidirectional connection path is first set up in the forward direction and then a further unidirectional connection path is set up in the backward direction. Both forward and backward directions of a bidirectional connection path usually take the same route, or the same optical fiber path, in the optical switched transmission system. In each case, the same wavelength is typically used for a unidirectional optical connection path (“link”) for the forward and backward directions. In the connection path setup described in the publication, the setting up of a bidirectional connection path is only possible with considerable additional signaling complexity. For example, in the method in question, it is necessary to ensure that, during the setting up of the forward direction, the resources necessary for the backward direction, such as optical fiber link, wavelength, etc., within the optical transmission system are in no way enabled for the setting up of further connection paths, but rather are reserved for the current optical connection. The two-stage character of the method described leads to connection setup times which are approximately twice as long as those for the connection setup of a unidirectional connection. A further disadvantage of such a method for setting up a bidirectional connection path is the significantly longer time required for the connection setup.
In addition, a one-stage method for setting up a bidirectional connection path is known from the publication “Extensions to RSVP-TE for Bi-directional Optical Path Setup”, by Don Guo et al., Network Working Group, January 2001. Here, the wavelengths which are used for setting up the forward and backward directions for a bidirectional connection path can be selected as desired so that the setting up of the bidirectional connection path can be carried out approximately simultaneously. However, in this method it is not ensured that, in each case, the same wavelength is provided for setting up the forward and backward directions of an optical bidirectional connection path. In the method in question, the forward and backward directions each disadvantageously take separate routes (i.e., are routed via different optical connection paths or optical fibers), in the optical WDM transmission system. As a result, in the event of a fault, the fault diagnosis or the fault localization is made considerably more difficult.
An object of the present invention, therefore, is to provide an optical transmission system and a method for setting up bidirectional connection paths in this optical transmission system with a number of switched optical network nodes which simplifies the setting up of bidirectional connection paths.